fix: more efficient template effect grouping

[Rich-Harris]

This simplifies the compiler code around template effects and makes the output more readable and concise. Currently we often generate a bunch of template effects per fragment, when in almost all cases we need zero or one.

It also deduplicates repeated expressions, since there's no point running a pure function twice in the same moment.

A handful of tests needed to be updated because this changes the render order in a way that doesn't matter (i.e. text nodes and attributes in a given fragment are always updated at the end, even if they come before children in the DOM).

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 34ad652

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-15050-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15050

[trueadm]

Why is the parent template now happening before all the children templates?

[Rich-Harris]

It's the opposite. Previously you could have multiple template effects in the root fragment, and the {logRender()} call happened before rendering the child. Now, there's just one template effect, at the end (after rendering child blocks and components etc). I don't consider this a breaking change since we're already grouping some effects at the end without giving you any control over it, and since template effects should be pure it's effectively not observable

[trueadm]

LGTM